The information described in this background section is not admitted to be prior art.
Filters generally operate in one of two modes: dead-end or cross-flow. In dead-end filtration, the feed fluid to be filtered flows in a direction generally perpendicular to the surface of the filtration medium, for example, a semi-permeable membrane or a granular media bed. Dead-end filtration can be effective in applications with low concentrations of particles or other materials to be removed from the feed fluid or in applications where the packing tendency of the material retained by the filtration medium (i.e., the retentate) does not produce a large pressure drop across the filter medium. Typical industrial uses of dead-end filtration include municipal water treatment and food and beverage applications such as the filtration of beer, wine, and other beverages.
Many industrial process streams contain high concentrations of particles, solutes, or other materials to be removed which, in many cases, rapidly foul dead-end filtration media, thereby increasing back pressure and decreasing filtration rate. Dead-end filtration techniques are therefore industrially impractical in such applications. However, cross-flow filtration techniques can be employed in such applications to maintain industrially acceptable filtration rates and periods of operation. In cross-flow filtration, the feed fluid to be filtered flows in a direction generally parallel to the surface of a semi-permeable membrane—i.e., tangentially to the filtration membrane. As a portion of the feed fluid passes through the filtration membrane and becomes the permeate stream, particles, solutes, or other materials are concentrated in the feed fluid on the feed side of the filtration membrane which becomes the retentate stream.
Cross-flow filtration operates according to theoretical principles derived from Fick's law of diffusion. The feed stream flows through a cross-flow filter at a pressure, concentration, or other physical or chemical differential between the feed/retentate stream and the permeate stream on the opposite side of the filtration membrane. Particles, solutes, or other materials which are smaller than the filtration membrane pore size pass through the membrane as filtrate in the portion of the feed fluid that forms the permeate stream. Some of the particles, solutes, or other materials in the feed stream become trapped in or on the filtration membrane as retentate, while the balance of the feed stream flows across the filtration membrane on the feed side, without passing through the membrane or becoming trapped in or on the membrane, and forms the retentate stream. The retentate stream exiting a cross-flow filter (i.e., the unfiltered portion of the feed stream) is maintained separate from the permeate stream exiting the cross-flow filter, and may be recycled back through the filter, fed to separate downstream filters for additional filtration, fed to other unit operations, or collected, as appropriate for particular applications.
Cross-flow filters may be used to filter feed streams containing particles, solutes, or other materials that would rapidly “blind” or otherwise foul dead-end filters. “Blinding” is an accumulation of retentate on a filtration membrane that fouls and/or reduces the effectiveness of a filter. In cross-flow filtration, the tangential motion of the bulk fluid across the filtration membrane can mechanically dislodge retentate materials from the membrane surface and the tangential feed flow can transport the dislodged and/or otherwise concentrated retentate materials out of the filter. Consequently, a cross-flow filter can operate in a continuous mode for long periods of operation with decreased blinding at relatively high solids load compared to dead-end filters.
Cross-flow filtration membranes can be produced in tubular, flat sheet, spiral-wound, and hollow fiber configurations. Additionally, cross-flow filtration membranes can be produced with pore sizes ranging from less than 10 Angstroms (reverse osmosis membranes) to greater than 10 micrometers (conventional filtration membranes), and include nanofiltration membranes 1-10 nanometer pore sizes), ultrafiltration membranes (˜10-100 nanometer pore sizes), and microfiltration membranes (˜0.1-10 micrometer pore sizes).